1. Field of the Invention
The present invention is directed to a drive circuit which generates a pulse for an electromagnetic source for generating shockwaves of the type including a dischargeable high-voltage capacitor.
2. Description of the Prior Art
A known circuit of the above type is shown in FIG. 1. The circuit has a constant voltage source 1, a switch 2, which is usually implemented as a spark gap, a high-voltage capacitor 3 as well as a coil, which is part of an acoustic wave-generating unit of an electromagnetic shockwave source. In addition to the coil 4, the acoustic wave-generating unit of the electromagnetic source has a coil carrier (not shown) on which the coil 4 is arranged and a magnetically susceptible membrane (likewise not shown) arranged on the coil 4 with an intervening insulator. Upon discharge of the high-voltage capacitor 3 across the coil 4, a current I flows through the coil 4, and the resulting electromagnetic field interacts with the membrane. The membrane is thereby repelled into an acoustic propagation medium, as causing pressure waves to be emitted into the acoustic propagation medium as a carrier medium between the acoustic wave-generating unit of the electromagnetic source and a subject to be acoustically irradiated. Shockwaves can arise from the acoustic source pressure waves due to non-linear effects in the carrier medium. The structure of such an electromagnetic shockwave source is disclosed in European Application 0 133 665 as an example.
When the circuit shown in FIG. 1 is operated for generating acoustic waves, then the curves of the voltage U, of the current I and of the square of the current I (I2) shown as examples in FIG. 2 occur during the discharge event of the high-voltage capacitor 3 across the coil 4, upon the circuit being shorted by means of the switch 2. Curve 5 shows the voltage curve across the coil 4, and curve 6 shows the decaying current I flowing through the coil 4, this current I, as already mentioned, being the cause for generating acoustic waves. Curve 7 illustrates the square of the current I.
The acoustic waves generated by the electromagnetic shockwave source are proportional to the square of the current I. As can be seen from FIG. 2 on the basis of the square of the current I, a first acoustic source pressure wave from the first acoustic pressure pulse (1st maximum) and further acoustic source pressure waves from the decaying sequence of positive acoustic source pressure pulses, proceed from a discharge event of the high-voltage capacitor 3. Due to non-linear effects in the carrier medium and a non-linear focusing, which usually ensues with a known acoustic focusing lens, the first source pressure wave and the following source pressure waves can form into shockwaves with short, intensified positive components and subsequent, elongated, under-pressure troughs, as already mentioned.
Shockwaves are utilized, for example, for non-invasive disintegration of calculi from the onside of the body of a patient, for example for destroying a kidney stone. The result of the shockwaves directed onto the kidney stone is that cracks arise in the kidney stone. The kidney stone ultimately breaks apart and thus can be eliminated in a natural way.
The first shockwave that proceeds from the first source pressure pulse is the determining factor for the concretion-disintegrating growth of cracks in the stone. Due to the greatly increased proliferation rate caused by the under-pressure trough of the first shockwave, the following shockwaves lead to only superficially acting cavitation processes that can even be harmful to tissue.